The present invention relates to an explosion-proof film good in visibility, scratch-resistance, and workability, which is stuck on a display screen of a cathode-ray tube.
To prevent scattering of glass due to implosion of a cathode-ray tube, there has been proposed a method of sticking an explosion-proof plastic film on a display screen of the cathode-ray tube. The sticking of such an explosion-proof film on the display screen of the cathode-ray tube is effective to reduce the thickness of a panel glass and hence to contribute to lightweightness of the cathode-ray tube, because the explosion-proof film shares a function of preventing scattering of glass which has been dependent on the panel glass or a tension band.
With respect to a transparent material through which a substance is to be viewed, when light is intensively reflected from the surface of the transparent material or when an image is clearly formed on the surface thereof, it becomes very difficult to view the substance through such a transparent material. For example, in the of spectacle lenses, a reflected image called "ghost" or "flare" formed thereon gives discomfort to eyes, and in the case of looking glass, reflected light on the glass surface obstructs clear viewing of a substance. Such a phenomenon also occurs for a panel glass of a cathode-ray tube, and to cope with such an inconvenience, various countermeasures have been proposed.
As one example of the countermeasures, there has been known a method of preventing reflection of light from the surface of a base member by coating the surface of the base member with a material having a refractive index different from that of the base member by vacuum deposition or the like. In this method, to improve the reflection preventive effect, it is important to control the thickness of the coating material. In the case where a single layer film is used as the coating material, the minimum reflectance, that is, the maximum transmittance is obtained by forming the film using a material having a refractive index lower than that of the base member and selecting an optical film thickness of the material to be equal to a quarter-wavelength of light or the quarter-wavelength multiplied by an odd number. The optical film thickness is given by a product of the refractive index of the film forming material and the thickness of the film.
As the material for forming a reflection preventive film, there is generally used a material exhibiting a low reflectance and a high transmittance of visible rays, which is represented by an inorganic oxide or an inorganic halide. Further, there are proposed several methods of forming a reflection preventive film having a plurality of layers.
In general, the transmittance of a display screen of a cathode-ray tube is adjusted by a panel glass; however, in the case of sticking an explosion-proof film on the panel glass for reducing the thickness of the panel glass and achieving lightweightness of the cathode-ray tube, the transmittance of the display screen is increased, resulting in the reduced contrast. In particular, in the case of sticking a film with a reflection preventive film made from an inorganic oxide on the panel glass, the reflection preventive film exhibits little light absorption and the degree of reflection on the surface is reduced, so that the actual transmittance is further increased, thereby further reducing the contrast.